1. Field of the Invention
The field of the present invention relates to wireless communication and, more particularly, to paging techniques in a wireless communication system.
2. Description of Related Art
A mobile communication system may generally comprise a set of xe2x80x9cuser stations,xe2x80x9d typically mobile and the endpoints of a communication path, and a set of xe2x80x9cbase stations,xe2x80x9d typically stationary and the intermediaries by which a communication path to a user station may be established or maintained. A group of base stations may be connected to a base station controller, or a cluster controller, which can in turn be connected to a local public telephone network through, for example, a mobile switching center.
It is generally desirable in a mobile communication system to achieve the greatest possible user traffic capacity at a base station, so that fewer base stations need to be deployed in order to serve user demands. A variety of techniques have been developed or proposed by which a base station is able to communicate with multiple user stations. Such techniques generally each include some means for distinguishing transmissions between different cells and/or between different user stations within a cell. For example, a communication system in which transmissions are distinguished according to the transmission frequency may be referred to as a frequency division multiple access (FDMA) communication system. A communication system in which a forward link transmission over one frequency is paired with a reverse link transmission over a different frequency may be referred to as a frequency division duplex (FDD) communication system. A communication system in which transmissions are distinguished according to the relative timing of the transmission (i.e., by use of time slots) may be referred to as a time division multiple access (TDMA) communication system. A communication system in which a forward link transmission during one time slot (or time segment) is paired with a reverse link transmission occurring during a different time slot (or time segment) may be referred to as a time division duplex (TDD) communication system. A communication system in which transmissions are distinguished according to which code is used to encode the transmission may be referred to as a code division multiple access (CDMA) communication system.
In a CDMA communication system, the data to be transmitted is generally encoded in some fashion, in a manner which causes the signal to be xe2x80x9cspreadxe2x80x9d over a broader frequency range and also typically causes the signal power to decrease as the frequency bandwidth is spread. At the receiver, the signal is decoded, which causes it to be xe2x80x9cdespreadxe2x80x9d and allows the original data to be recovered. Distinct codes can be used to distinguish transmissions, thereby allowing multiple simultaneous communication, albeit over a broader frequency band and generally at a lower power level than xe2x80x9cnarrowbandxe2x80x9d systems. Different users may thereby transmit simultaneously over the same frequency without necessarily interfering with one another.
In addition to the above, various xe2x80x9chybridxe2x80x9d communication systems incorporating aspects of more than one multiple access communication technique have been developed or proposed.
Initial communication between a user station and a base station can be established either when the user station seeks to initiate communication with a base station (for example, attempting to initiate a telephone call), or when the base station attempts to complete a call to the user station (for example, where the user station is paged). In many conventional mobile communication systems, a dedicated control channel is used to assist mobile stations in establishing communication. According to this technique, the mobile station first communicates over the control channel when establishing communication. The base station then assigns to the mobile station a xe2x80x9cpermanentxe2x80x9d communication channel for exchanging bearer traffic messages for the duration of the call. Particular techniques for establishing initial communication between a base station and a user station are described, for example, in U.S. Pat. No. 5,455,822, U.S. Pat. No. 5,737,324, U.S. Pat. No. 5,671,219, U.S. Pat. No. 5,648,955, and U.S. Pat. No. 5,787,076, each of which is assigned to the assignee of the present invention, and each of which is hereby incorporated by reference as if set forth fully herein.
When a base station pages a user station, the base station typically sends a paging message directed to the specific user station (e.g., by including a specific user station identifier, which may be obtained from the user station at initial registration). Often a special broadcast channel is reserved for this paging purpose, sometimes in conjunction with additional broadcast functions. When the user station receives a paging message, it responds according to the particular communication protocol employed by the system, and thereby receives the incoming call. If more than one paging request is received at the base station, the base station may transmit multiple paging messages sequentially over the broadcast channel. The base station may repeat a paging message, or a group of paging messages, until each user station responds or a paging timeout occurs.
It has been found convenient in many mobile communication systems to allow user stations (particularly cellular telephone handsets) to remain in a low-power, dormant state (i.e., a sleep state) while no call is in progress, becoming active periodically (i.e., awakening) only long enough to determine whether it is being paged. In a typical mobile communication system, a handset awakens or otherwise activates at regular, fixed intervals to monitor the broadcast paging channel from the base station. If the handset receives a paging message during the time it is awake or active, it does not return to sleep or dormancy, but instead responds to the paging message according to the system protocol in an attempt to receive the incoming call. On the other hand, if no paging message for the handset is received, the handset returns to a sleep or dormant state.
In most mobile communication systems, paging is only one of a number of broadcast functions carried out by the base station. Where multiple broadcast functions are performed by the base station, the base station broadcast channel may be shared between paging functions other broadcast functions by, for example, time multiplexing paging messages and other messages. A result is that paging messages may only be sent periodically by the base station. However, if the handset wakes up and monitors the broadcast channel at the wrong times (i.e., at times other than when paging messages are being broadcast from the base station), the handset will miss any paging messages directed to it. One solution to this problem is to define a preset time slot or time interval for broadcasting paging information from the base station. Once the handset locks on to the paging channel, it wakes up at regular intervals matched to the transmission repetition rate of the paging channel. The handset thereby has an increased likelihood of receiving a page, since it will monitor the paging channel only at times when paging information is being transmitted. As a benefit of such an approach, the time which a handset can spend asleep or dormant is increased (thus saving power and increasing battery life), because there is a general assurance that the handset will not miss a page when the handset is asleep or dormant, particularly if the page is repeated a reasonable number of times.
While the above solution may increase power savings at the handset, it is not a totally adequate solution. It suffers from the drawback that the same amount of base station resources are consumed with paging traffic no matter whether the paging traffic is heavy or light. Reserving resources for paging traffic prevents the use of such resources for other broadcast needs. If too many resources are reserved for paging at a given time, the system is inefficient. Conversely, when paging traffic is heavy, the reserved amount of broadcast resources for paging may be insufficient, and calls may not get completed timely or may be entirely missed. Moreover, if user stations are required to monitor the paging channel too frequently, the user stations use more power than they otherwise would, leading to wasted energy and/or battery life.
The problem of efficient paging traffic may be particularly acute in a wireless packet-oriented communication system. Often, each packet transmission from the base station requires a separate page. Where a large number of packets are transmitted from the base station, the paging overhead can be substantial, and inefficiencies in allocation of paging resources can substantially impact the system performance.
It would therefore be advantageous to provide a flexible paging technique for a mobile communication system in which paging resources are allocated in a manner more closely related to the actual paging traffic.
The invention provides in one aspect a flexible paging protocol which adjusts the allocation of resources for paging in response to the amount of actual paging traffic. Resources for paging are increased when paging traffic is heavy, and decreased when paging traffic is light. The user stations (e.g., mobile end systems) are preferably notified of the allocation of paging resources dynamically at each paging period.
In a preferred embodiment, a base station communicates with a plurality of mobile end systems which may be paged for incoming calls. The base station periodically transmits over a paging broadcast channel, which may be time-multiplexed with other channels. The periodicity of the paging channel is dynamically adjusted based on the amount of paging traffic. A paging packet message transmitted by the base station preferably comprises a set of paging messages and a next page pointer. The next page pointer identifies when the next set of paging messages (i.e., the next paging packet message) will be transmitted. A mobile end system monitoring the paging channel receives the next page pointer, and, if not being paged, sets its sleep or dormancy period based upon the amount of time remaining until the next paging packet message is transmitted by the base station.
When paging traffic is light, paging messages are sent infrequently, freeing up base station resources and allowing the mobile end system to remain asleep or dormant longer. When paging traffic is heavy, paging messages are sent more frequently, ensuring rapid response to the pages and reducing the probability of a missed call.
In one embodiment as disclosed herein, a base station communicates with a plurality of user stations using time division multiple. access (TDMA) techniques. The base station generates a repeating time frame comprising a plurality of time slots. One of the time slots (or possibly a time slot on another frequency) may be reserved for broadcast or other control traffic. Several logical channels, including a paging channel, may be multiplexed on to the same physical broadcast/control channel. When the paging channel is active, the base station may broadcast one or more paging messages, in the form of one or more paging packet messages, to mobile end systems monitoring the paging channel. As part of a paging packet message, the base station transmits a next page pointer, which preferably identifies the number of time frames until the next paging packet, and therefore the amount of time which will elapse until the mobile end systems need to next wake up to listen for pages. In a particular embodiment, the next page pointer comprises a set of N bits which together define 2N different possible paging intervals until the next paging packet. For example, if N is 3 bits, a total of 8 different possible paging intervals until the next paging packet can be defined. The next page pointer can be encoded, so as to allow for a wide variation in the paging interval. For example, the next page pointer may be encoded such that 8 possible paging interval indicators correspond to intervals of 1, 2, 4, 8, 16, 32, 64 and 128 time frames between paging packets.
The next page pointer may be used in a system which employs a next slot pointer for increasing the speed of control traffic transactions. In such an embodiment, the message bits in a control traffic message ordinarily reserved for identifying the. next slot of a control traffic message are interpreted as identifying the time interval until the next paging packet is received.
A preferred embodiment disclosed herein is particularly well suited for a wireless packet-oriented communication system. In this embodiment, the base station receives data packets over a backhaul connection for transmission to mobile end systems. The base station transmits a paging message to each mobile end system for which a data packet has been received. When the mobile end system responds, the base station divides the data packet into a sequence of over-the-air packets, and transmits the over-the-air packets to the mobile end system in assigned time slots.
A technique for flexible paging is disclosed with respect to a preferred embodiment wherein the base station and mobile end systems communicate in TDMA time slots using spread spectrum encoded messages, and wherein data symbols are encoded using an M-ary direct sequence spread spectrum communication technique. Further variations and details of the above embodiments are also described herein and/or depicted in the accompanying figures.